A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as well cementing. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
Subsequently, oil or gas residing in the subterranean formation may be recovered by driving the fluid into the well using, for example, a pressure gradient that exists between the formation and the wellbore, the force of gravity, displacement of the fluid using a pump or the force of another fluid injected into the well or an adjacent well. The production of the fluid in the formation may be increased by hydraulically fracturing the formation. That is, a viscous fracturing fluid may pumped down the casing to the formation at a rate and a pressure sufficient to form fractures that extend into the formation, providing additional pathways through which the oil or gas can flow to the well. Unfortunately, water rather than oil or gas may eventually be produced by the formation through the fractures therein. To provide for the production of more oil or gas, a fracturing fluid may again be pumped into the formation to form additional fractures therein. However, the previously used fractures first must be plugged to prevent the loss of the fracturing fluid into the formation via those fractures.
In addition to the fracturing fluid, other fluids used in servicing a wellbore may also be lost to the subterranean formation while circulating the fluids in the wellbore. In particular, the fluids may enter the subterranean formation via depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the drilling fluid, and so forth. As a result, the service provided by such fluid is more difficult to achieve. For example, a drilling fluid may be lost to the formation, resulting in the circulation of the fluid in the wellbore being too low to allow for further drilling of the wellbore. Also, a secondary cement/sealant composition may be lost to the formation as it is being placed in the wellbore, thereby rendering the secondary operation ineffective in maintaining isolation of the formation.
The isolation of the subterranean formation may be compromised by permeable zones present in the annulus and/or the wall of the casing in the wellbore. Permeable zones such as voids in the cement column placed in the annulus may result from an incomplete fill in of the annulus during primary cementing. Permeable zones may also be caused by the expansion and contraction of the casing and the cement column due to cyclical changes in underground pressures and temperatures during the life of the well. Moreover, the cement column may experience impacts and shocks generated by subsequent drilling, pressure testing, hydraulic fracturing, or other well operations. Unfortunately, conventional cement suffers from the drawback of being brittle and fragile and thus often cannot sustain such stress. Consequently, cracks or voids may form in the cement column and/or the casing wall. Further, microannuli may form between the cement column and the casing and between the cement column and the subterranean formation. Fluids may undesirably migrate through the microannuli, the voids or cracks in the cement column, and/or the voids or cracks in the casing wall such that the subterranean formation is no longer isolated from other subterranean formations.
Accordingly, a need exists to develop methods for maintaining the isolation of a subterranean formation after the completion of an adjacent wellbore and for reducing the loss of fluid into the formation. It is therefore desirable to devise ways of blocking the flow of fluid through pathways such as fractures filled with water, loss circulation zones in the subterranean formation, voids or cracks in the cement column and the casing, and so forth.